The present invention relates to communications systems in general and, more particularly, to a method and apparatus for smoothly transmitting bursty data in a wireless communications system.
Existing CDMA (code-division multiple access) systems based on TIA/EIA standard IS-95 support the transmission of digital information at rates of up to 9.6 kbps or at most 14.4 kbps per channel. However, these data rates are considered to be insufficient for supporting many emerging data transfer applications, including browsing of the World Wide Web, electronic mail, e-commerce, tele-medicine and the like. With the goal of providing greater flexibility in the delivery of data services while continuing to exploit the features and merits of a CDMA-based system, various North American, European and Japanese consortia have developed proposals for what has become known as third generation (3G) CDMA.
As a consequence of 3G CDMA being able to support a wide range of data rates, links having different data rates may vary considerably in the power they consume. For example, to maintain the same quality of service for a 384 kbps link, approximately 16 dB more power is required than for a 9.6 kbps link. (The latter is typical of the bit rate of a standard voice channel in both IS-95 and 3G CDMA, while the former is typical of the bit rate that has been proposed for use by high-speed data channels in 3G CDMA.) Thus, the abrupt start of a 384 kbps transmission is roughly equivalent to the simultaneous origination of 40 voice calls.
The interference caused by establishing high-speed data connections depends on various factors, such as the cell(s) in which users are located, the direction of communication (forward-link or reverse-link) and the cell loading. By way of illustration, let there be a high-speed link established between a first mobile unit and a base station in a cell C1. If a second mobile unit is also located in cell C1 and if the first mobile unit""s high-speed connection is in the forward-link direction, then very little interference will be felt by the second mobile unit, due to mutual orthogonality of the signals transmitted by the base station in cell C1.
On the other hand, if the second mobile unit is located in a cell C2 which borders with cell C1 and if the first mobile unit""s high-speed connection has been established in the reverse-link direction, then factors such as the loading of cell C2 and the proximity of the second mobile unit to the boundary of cell C1 will determine the level of interference felt by the second mobile unit. This can range from very mild (when cell C2 is heavily loaded and the second mobile unit is far away from the boundary between cells C1 and C2) to very severe (when cell C2 has very few active calls and the second mobile unit is proximate the boundary between cells C1 and C2).
An increase in the interference felt by other users (e.g., the second mobile unit in the above example) results in a degradation in the quality of service of the connections established by those other users. Consequently, the transmitted power of the affected links will be increased using standard power control algorithms until an acceptable quality of service is again attained. However, the other users may not be able to increase their power levels quickly enough, which may result in data being lost or delayed in the meantime.
Furthermore, even if the other users are capable of raising their power levels quickly, a certain amount of time will elapse before stable power levels are arrived at by the various users. This is due to the fact that when link power is increased to satisfy any one user, interference will be caused to the remaining users, who then have to raise their respective link power, thereby affecting the user who originally required an increase in link power, and so on. This chain reaction continues until all users reach a stable power level but in the meantime, some users may experience a lower signal quality than required. Depending on the degree of burstiness of the high-speed data, such chain reactions may be initiated many times per second and thus the power levels may not be given a chance to converge.
Even more fundamentally, the interference caused by a bursty high-speed data transfer involving a particular user will tend to decrease during gaps between bursts. During such gaps, additional calls may be admitted by the system controller. However, the admission of calls based on a low interference level at the time of admission may cause signal degradations to occur when the high-speed link becomes active at a later time, e.g., during a subsequent burst.
Moreover, if the quality requirement for the high-speed link is greater than that for voice, the impact of turning on that high-speed link is even more dramatic. Similarly, the effects of interference are magnified when multiple high-speed links are established.
Clearly, a major problem facing the developers of 3G CDMA systems is that of addressing or tempering the sudden interference changes resulting from the bursty nature of high-speed data transmissions.
The invention may be summarized according to a first broad aspect as a method of smoothing the bit rate transitions in a bursty input data stream. The method includes the steps of receiving the input data stream in a buffer, periodically measuring the occupancy level of the buffer and withdrawing the contents of the buffer at an output rate. The output rate is made dependent on the buffer occupancy level, which allows the output data stream to have smoother data rate transitions than does the input data stream.
In a simple embodiment of the invention, the output rate is increased if the buffer occupancy level is above a threshold occupancy level and decreased if the buffer occupancy level is below a second threshold level, where both threshold occupancy levels are dependent on the current output rate.
In the preferred embodiment of the invention, the output rate is increased if any of the following is true: (1) the most recent change to the output rate was a decrease AND the buffer occupancy level is above a first threshold occupancy level dependent on the current output rate; (2) the most recent change to the output rate was an increase AND a predetermined amount of time dependent on the output rate has elapsed since the most recent change to the output rate AND the buffer occupancy level is above a second threshold occupancy level dependent on the output rate; (3) the output rate is initially at a predetermined minimum value AND the buffer occupancy level is above a third threshold occupancy level.
Analogously, the output rate is preferably decreased if any of the following conditions is met: (1) the most recent change to the output rate was an increase AND a first predetermined amount of time dependent on the output rate has elapsed since the most recent change to the output rate AND the buffer occupancy level is below a first threshold occupancy level dependent on the current output rate; (2) the most recent change to the output rate was a decrease AND a second predetermined amount of time dependent on the output rate has elapsed since the most recent change to the output rate AND the buffer occupancy level is below a second threshold occupancy level dependent on the output rate; (3) the output rate is at a predetermined maximum value AND the buffer occupancy level is below a third threshold occupancy level.
According to another broad aspect, the invention may be summarized as a data rate smoothing unit. The smoothing unit has a buffer for receiving a bursty input data stream and a processing unit connected to the buffer, for withdrawing data from the buffer at a controllable output rate. The buffer is operable to periodically measure its occupancy level. The smoothing unit also has a controller connected to the buffer and to the processing unit. The controller is operable to vary the output rate as a function of the occupancy level to provide an output data stream with a less bursty nature than the input data stream.
The invention may be summarized according to yet another broad aspect as a CDMA transmission system having a data source, a bit rate control module, a data channel encoder and a controller. The data source provides an input data stream consisting of data bursts. The bit rate control module receives the input data stream and is operable to periodically measure the occupancy level of the buffer. The data channel encoder is connected to the bit rate control module and withdraws from it an output data stream at a controllable output rate and performs error correction on the output data stream. Finally, the controller is connected to the bit rate control module and to the data channel encoder, and is operable to vary the output rate as a function of the comparison of the occupancy level to a threshold occupancy level, resulting in the output data stream having fewer bursts than the input data stream.
A transmitter may also be provided in the transmission system, for scaling the output of the data channel encoder in accordance with a controllable output power level. This output power level is set by the controller and is preferably proportional to the output rate. The transmitter may also be equipped with modules for performing pseudo-random noise spreading of the control signal and the output of the data channel encoder, combining the spread signals into a composite signal, amplifying the composite signal and transmitting the amplified signal over a wireless link.
The transmission system may be part of either a mobile unit or a base station. If it is part of a base station, there may be an individual system for each of a plurality of users, running in parallel with one another.
As a result of smoothing the bursty data stream, there is little degradation of service quality to other users of the system upon establishing the bursty connection and problems related to convergence of power levels across the entire system are practically eliminated. Furthermore, power amplifier requirements in the transmitter may be relaxed, which can prevent outages of the high-speed link itself. Also, since there are fewer abrupt changes in the output rate, there will be fewer abrupt changes in the induced interference, leading to a higher percentage of call admissions which are retained and hence the capacity is increased.